It is noted that SDLs are also known in the art as Vertical External Cavity Emitting Lasers (VECSELs) or Optically Pumped Semiconductor Lasers (OPSLs). Therefore the term semiconductor disc laser (SDL) when used throughout the present description is used to refer to each of these systems.
The term “ultra short” pulses as used within the following description refers to pulses having a duration from about 100 picoseconds (ps) down to a few femtoseconds (fs).
Ultra short pulses of optical radiation generated by laser sources are employed in a range of scientific, instrumentation and nonlinear optics applications. A well-known technique for generating short or ultra short pulse generation is mode locking. When mode locking takes place in a laser resonator a plurality of modes of the laser resonator are coupled in a phase-locked manner in such a way that the resulting generated electromagnetic field comprises a short pulse or cavity mode which circulates within the resonator. It can be induced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator within a particular time window.
The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronised to the cavity roundtrip time. However, ultra short pulse generation relies on passive mode-locking techniques, because only a passive shutter is fast enough to shape and stabilise ultra short pulses. Passive mode locking generally relies on a saturable absorber mechanism, which produces decreasing loss with increasing optical intensity. When the saturable absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking can be achieved.
It is known in the art to employ a saturable Bragg reflector (SBR) to passively mode lock a solid state laser, see for example Tsuda et al “Mode-Locking Ultrafast Solid-State Lasers with Saturable Bragg Reflectors”, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 3, September 1996 pp. 454-463 and U.S. Pat. No. 5,627,854. An SBR is a nonlinear mirror that comprises one or more semiconductor quantum wells within a standard distributed Bragg reflector (DBR) i.e. a stack of alternating layers of quarter wave semiconductor materials. As result, the reflectivity, or absorption, exhibited by an SBR is intensity dependent i.e. the reflectivity is higher for higher light intensities. Femtosecond mode locking of resonators containing Ti:sapphire or Cr:LiCAF solid state gain mediums have been demonstrated employing these techniques.
US patent publication number 2004/0190567 and U.S. Pat. No. 6,735,234 B1 extend the above concepts so as to mode lock an SDL by incorporating a saturable Bragg reflector (SBR) within the resonator. U.S. Pat. No. 6,735,234 B1 discloses that during operation, a coupled cavity effect forms between the Bragg reflector of the SDL and Fresnel reflections from the front surface of the gain medium of the SDL. As this sub-cavity operates near resonance at the laser wavelength it acts to lower the effective gain saturation of the laser to a value well below the absorption saturation of the SBR. In order to compensate, and hence raise the gain saturation level of the device it is disclosed that the optical resonator design should be chosen to ensure that the cross sectional area of the intra cavity resonating field on the gain medium is larger than the cross sectional area of the intra cavity resonating field on the SBR.
In a number of scientific, instrumentation and nonlinear optics applications it is often desirable to obtain as stable a mode-locked laser output as possible. It is therefore recognised that it would be advantageous to provide a passively mode-locking semiconductor disc laser system that exhibits increased stability when compared to those passively mode-locking semiconductor disc laser systems known in the art.